The Swan-Ganz catheter provides useful hemodynamic data that can guide therapy of critically ill patients. Nurses in many areas of practice must be familiar with this method of monitoring. The nurse is responsible for calibrating the equipment measureing and recording pressures as indicated. She is also responsible for using pressure measurements as vital data in assessing the patient's condition and in evaluating response to therapy. It is common practice to position the patient supine and flat, and to disconnect the patient who is on intermittent positive pressure ventilation from the ventilator for pulmonary artery and pulmonary capillary wedge pressure measurements. These practices are thought to give more valid measurements. Measurements are recorded hourly or more often depending upon the condition of the patient. This may require waking the acutely ill patient to reposition him flat, and also, there are patients who should not be placed flat in bed. Disconnecting the patient from the ventilator may also disturb a patient, increase his anxiety and expose him to potential hypoxia during the procedure. It is unknown whether accurate PA and PCW pressures can be obtained if the patient is not positioned flat and not disconnected from the ventilator. Recent preliminary work questions the necessity for these practices during pressure measurements. Further data, collected under controlled conditions with use of specialized equipment, are needed to answer these questions, If no difference is found, then accurate pressure measurements could be obtained without disturbing the acutely ill patient by positioning him flat and disconnecting him from the ventilator.